1. Field of the Invention
The present invention relates to an ultrasonic endoscope capable of being inserted into a body of a patient and imaging ultrasonic tomographic images for medical diagnoses and an ultrasonic endoscopic apparatus including such an endoscope.
2. Description of a Related Art
Recent years, an ultrasonic endoscope has been used for medical diagnoses by inserting it into a body of a patient and imaging ultrasonic tomographic images. In such an ultrasonic endoscope, generally, a mechanical radial scanning method of performing scanning with viewing angle of 360° by mechanically rotating an array including plural ultrasonic transducers (ultrasonic vibrators) that transmit and receive ultrasonic waves has been adopted.
However, according to the mechanical radial scanning method, compared to a scanning method of performing scanning with viewing angle of 90° using a convex ultrasonic transducer array in an external ultrasonic imaging apparatus, in the case where conditions of sound ray density, scanning depth, etc. are set into the same condition, there is a problem that scanning time (frame period) for obtaining an image for one frame becomes longer in order to perform scanning with wide viewing angle and the frame rate becomes low.
On the other hand, an electronic radial scanning method of performing scanning with viewing angle of 360° by electronic scanning has been proposed. For example, Kinichi Takeda et al., “The Experience of Electric Radial Endoscopic Ultrasonography on Gastroenterology”, Jpn J Med Ultrasonics, Vol. 31, Supplement (2004), 77-C062, pp. 238 discloses results from studies on efficacy and problems of an electronic radial ultrasonic endoscope based on experiences of using the electronic radial ultrasonic endoscope for various digestive system diseases.
Further, Japanese Patent Application Publication JP-A-2-134142 discloses an electronic radial ultrasonic beam apparatus having improved resolving power in which a flexible substrate on which plural ultrasonic vibrators are provided in a line is formed in a cylindrical shape and an interval between ultrasonic vibrators in the joint part of the flexible substrate is set to an integral multiple of an arrangement interval of the respective vibrators. Further, the apparatus includes drive element switching circuits and delay lines for enabling scanning operation at an interval position between the ultrasonic vibrators in the joint part by driving ultrasonic vibrators adjacent to the interval position (page 2, right upper column, line 14 to left lower column, line 3; page 4, right lower column, lines 2-10; and FIG. 3).
However, in the ultrasonic endoscope according to the electronic radial scanning method, in order to obtain a B-mode ultrasonic tomographic image with viewing angle of 360°, it is also required to sequentially transmit ultrasonic beams (sound rays) SL1 to SL360 in a frame period T as shown in FIG. 11A while varying the positions of ultrasonic transducers to be used along a circumferential direction as shown in FIG. 11B. Accordingly, compared to the case of performing scanning operation with viewing angle of 90° using a convex ultrasonic transducer array, the frame period T becomes about four times and the frame rate is reduced to about one-quarter.
Further, in the electronic radial ultrasonic endoscope, in the case where bloodstream information is obtained by the Doppler method, in order to improve the S/N ratio of minute bloodstream information, plural pulses are sequentially transmitted in the same direction. For example, as shown in FIG. 11D, four ultrasonic beams SL1-1 to SL1-4 are sequentially transmitted in the first direction, and the transmission is repeated while varying the transmission direction, and finally, four ultrasonic beams SL360-1 to SL360-4 are sequentially transmitted in the 360th direction. In this case, compared to the case of imaging the B-mode tomographic image as shown in FIGS. 11A and 11B, the frame period T becomes about four times (4T) and the frame rate is reduced to about one-quarter as shown in FIG. 11C.